Methods and apparatus for predicting and monitoring performance of a coherent optical transceiver

ABSTRACT

In some embodiments, an apparatus includes an optical transceiver configured to be operatively coupled to a network. The optical transceiver includes a photo diode and a processor configured to be operatively coupled to the photo diode. The photo diode is configured to measure a receiver optical power (ROP) value and send the ROP value to the processor. The processor is configured to measure a bit error rate (BER) value of a digital modulated signal at an input port of the optical transceiver. The processor is also configured to determine an estimated optical signal noise ratio (OSNR) value at the input port of the optical transceiver based on the ROP value and the BER value. The processor is configured to send a signal indicating the estimated OSNR value such that a planned route is selected for sending data signals through within the optical transceiver based on the estimated OSNR value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/197,646, now U.S. Pat. No. 10,211,917, filed on Jun. 29, 2016, andentitled “Methods and Apparatus for Predicting and MonitoringPerformance of a Coherent Optical Transceiver”, the disclosure of whichis incorporated herein by reference in its entirety.

BACKGROUND

Some embodiments described herein relate generally to methods andapparatus for optical communication. In particular, but not by way oflimitation, some embodiments described herein relate to methods andapparatus for performance monitoring of coherent optical communicationsystem.

With the increase in the amount of data that needs to be communicated,optical communication systems need to evolve to operate at higher datarates. For instance, some recent optical communication systems operatein the 100 gigabits per second (Gbps) range. Most long-haul opticalcommunication systems use optical amplifiers to compensate for theattenuation introduced by optical fiber. While the optical signal isboosted, noise is also added, which can limit the transmission distance.Therefore, it is desirable to monitor the optical signal noise ratio(OSNR) of a coherent optical receiver. Known systems that can monitorthe performance of a coherent optical receiver, however, are expensiveand complex.

Accordingly, a need exists for improved and simplified methods andapparatus to monitor performance of a coherent optical receiver.

SUMMARY

In some embodiments, an apparatus includes an optical transceiverconfigured to be operatively coupled to a network. The opticaltransceiver includes a photo diode and a processor configured to beoperatively coupled to the photo diode. The photo diode is configured tomeasure a receiver optical power (ROP) value and send the ROP value tothe processor. The processor is configured to measure a bit error rate(BER) value of a digital modulated signal at an input port of theoptical transceiver. The processor is also configured to determine anestimated optical signal noise ratio (OSNR) value at the input port ofthe optical transceiver based on the ROP value and the BER value. Theprocessor is configured to send a signal indicating the estimated OSNRvalue such that a planned route is selected for sending data signalsthrough within the optical transceiver based on the estimated OSNRvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an optical transceiver, accordingto an embodiment.

FIG. 2 is a block diagram illustrating examples of photonics within apluggable optical module of an optical transceiver, according to anembodiment.

FIG. 3 shows a graph indicating examples of an original BER value and afitted BER value as a function of an original OSNR value at differentROP values, according to an embodiment.

FIG. 4 is a graph representing examples of the correlation between theBER vs OSNR curve and the BER vs ROP curve, according to an embodiment.

FIG. 5 shows examples of the comparison between the measurement resultof the BER vs OSNR curve and the prediction result of the BER vs OSNRcurve, according to an embodiment.

FIG. 6 shows examples of graphs of BER vs OSNR curves at different ROPvalues describing accuracy of the BER vs OSNR prediction, according toan embodiment.

FIG. 7 shows examples of graphs of Q² factor (in dB value) vs chromaticdispersion (CD) value of an optical transceiver, according to anembodiment.

FIG. 8 shows examples of graphs of the Q² factor (in dB value) vs DGDvalue of an optical transceiver at different ROP values, according to anembodiment.

FIG. 9 is a flow chart illustrating a method of an improved OSNRperformance measurement of an optical transceiver, according to anembodiment.

DETAILED DESCRIPTION

In some embodiments, an apparatus includes an optical transceiverconfigured to be operatively coupled to a network. The opticaltransceiver includes a photo diode and a processor configured to beoperatively coupled to the photo diode. The photo diode is configured tomeasure a receiver optical power (ROP) value and send the ROP value tothe processor. The processor is configured to measure a bit error rate(BER) value of a digital modulated signal at an input port of theoptical transceiver. The processor is also configured to determine anestimated optical signal noise ratio (OSNR) value at the input port ofthe optical transceiver based on the ROP value and the BER value. Theprocessor is configured to send a signal indicating the estimated OSNRvalue such that a planned route is selected for sending data signalsthrough within the optical transceiver based on the estimated OSNRvalue.

In some embodiments, when an optical transceiver is in a testing mode,such as during a manufacturing operation, a calibration operation, atrouble-shooting operation or an upgrading operation, the transmitter ofthe optical transceiver can be configured to be connected to thereceiver of the optical transceiver through one of an optical switchintegrated within the optical transceiver, or an external loop-backconnection. Such an optical switch and/or the loop-back connection canfacilitate the measurements of the receiver optical power (ROP) valueand the bit error rate (BER) value. Given the fact that the BER vs OSNRcurve and BER vs ROP curve can be intrinsically correlated, the BER vsOSNR curve can be predicted or estimated based on the measured BER vsROP curve. The predicted BER vs OSNR curve, a strong indication of theperformance of the optical transceiver, can therefore facilitate thedesign, manufacture, and maintenance of the optical transceiver.

As used in this specification, the singular forms “a,” “an” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, the term “an optical modulator” is intended to mean asingle optical modulator or multiple optical modulators. For anotherexample, the term “an optical transceiver” is intended to mean a singleoptical transceiver or multiple optical transceivers.

FIG. 1 is a block diagram illustrating an optical transceiver, accordingto an embodiment. The optical transceiver 101 can be any high data rate(e.g., 100 Gbps) optical transceiver such as a transceiver implementingintensity modulation with direct detection, e.g., a coherent opticaltransceiver, e.g., a coherent optical M-ary quadrature amplitudemodulation (M-QAM) transceiver, a coherent polarization-multiplexed (PM)M-QAM transceiver, and/or the like. In coherent optical communicationsystems with a coherent optical transceiver, both magnitude and phaseinformation are used to transmit and receive data such as forphase-shift keying modulation (e.g., BPSK, PM-BPSK, QPSK, PM-QPSK) orquadrature amplitude modulation (e.g., M-QAM, or PM-M-QAM).

The optical transceiver 101 can be a component of an opticalcommunication device or system (e.g., a network device) such as awavelength division multiplexing (WDM) system, including a densewavelength division multiplexing (DWDM) system. For example, a WDMsystem may include multiple slots reserved for multiple boards, such ashost board 103. Host board 103 is an example of a line card used in WDMsystems. Each host board 103 may receive one or more removable“pluggable” photonics module 105 to provide optical connectivity for oneor more optical fibers 104. Aspects of this disclosure, however, are notlimited to WDM systems.

The optical transceiver 101 can be operatively coupled to a network 190via optical fibers (e.g., 104). When the optical transceiver 201 is in anormal operating mode (not a testing mode), the optical transceiver 101can transmit optical signals to the network 190 via a TX optical fiber(e.g., 104) and receive optical signals from the network 190 via an RXoptical fiber (e.g., 104). Examples of the network 190 include, but notlimited to, a fiber-optic network (e.g., a local area network (LAN),metropolitan area network (MAN), wide area network (WAN), or a long-haulnetwork), or a converged network having functionalities of both awireless network and a wired network.

The optical transceiver 101 includes a host board 103 and a coherentoptical front-end 138. The coherent optical front-end 138 can include asubset of the components of the optical transceiver 101. For example,the coherent optical front-end 138 can include a pluggable opticalmodule 105 and the components that interconnect the pluggable opticalmodule 105 and the processor 117. The pluggable optical module 105 canbe operatively coupled to the host board 103. For illustration purpose,the pluggable optical module 105 is described here. Embodimentsdescribed here, however, are not limited to pluggable optical modulesand can also be applied to non-pluggable on-board optical components.

The host board 103 includes a processor 117 and a pluggable interface115 operatively coupled to the processor 117. The pluggable interface115 of the host board 103 is operatively coupled to the pluggableinterface 125 of the pluggable optical module 105. In oneimplementation, the pluggable optical module 105 can be removablycoupled to the host board 103. In another implementation, the pluggableoptical module 105 can reside on the host board 103 or can be hardwiredto the host board 103. The pluggable interface 115 can be an electricalinterface in the case of non-pluggable on-board optical components orphysical interface providing electrical connections between pluggableoptical module 105 and host board 103.

The processor 117 can be or can include a general purpose processor, afield-programmable gate array (FPGA), an application specific integratedcircuit (ASIC), a Digital Signal Processing (DSP) chip, a combinationthereof, or other equivalent integrated or discrete logic circuitry. Theprocessor 117 can include one or more analog-to-digital converters(ADCs) (not shown in the figure). The processor 117 can be configuredto, for example, write data into and read data from a memory, andexecute the instructions stored within a memory. The processor candetermine characteristics (e.g., BER) of the optical transceiver 101. Insome implementations, based on the methods or processes stored withinthe memory, the processor can be configured to execute an improvedperformance monitoring process, as described in FIG. 9. In someimplementations, the improved performance monitoring process asdescribed in FIG. 9 can also be executed in a processor (not shown inFIG. 1) of a layer higher than the layer of the host board 103, forexample, in a processor of a management and control layer of thewavelength division multiplexing system.

The pluggable optical module 105 includes a pluggable interface 125 andphotonics 127. The pluggable interface 125 can be an electricalinterface in the case of non-pluggable on-board optical components orphysical interface providing electrical connections between pluggableoptical module 105 and host board 103. Pluggable interface 115 of thehost board 103 and pluggable interface 125 of the pluggable opticalmodule 105 can mate with one another to couple pluggable optical module105 to host board 103. With pluggable interface 115 and pluggableinterface 125, pluggable optical module 105 can be selectively coupledto or decoupled from host board 103. Details of the pluggable opticalmodule 105 including the photonics 127 are described below with regardsto FIG. 2.

In the receiver direction, when the optical transceiver 101 is in anormal operating mode (not a testing mode), the pluggable optical module105 can receive an optical data signal from the network 190, convert theoptical data signal into electrical data signals, and output theelectrical signals to processor 117 via the pluggable interface 125 andpluggable interface 115. The processor 117 can further process theanalog electrical signals and reconstruct the data. In a WDM system,pluggable optical module 105 can receive higher data rate opticalsignals via optical fiber 104 from network 190, and convert the opticalsignals to electrical signals. Host board 103 can receive the electricalsignals from pluggable optical module 105, and host board 103 or theother board deserializes the electrical signals into lower data rateelectrical signals for transmission to the routers and switches(connected directly or indirectly to optical transceiver 101 and notshown in FIG. 1).

In the transmission direction, the processor 117 can send an electricaldata signal to the photonics 127 of the pluggable optical module 105 viathe electrical interface 115 and the electrical interface 125. Thephotonics 127 of the pluggable optical module 105 can convert theelectrical data signal to an optical data signal and further send theoptical data signal to the network 190 via the optical fiber 104 whenthe optical transceiver 101 is in a normal operating mode (not a testingmode). In a WDM system, a chassis (not shown) can house various types ofdevices such as routers, servers, and the like, and can include hostboard 103. Host board 103 or another board connected to host board 103receives lower data rate electrical signals from multiple devices suchas switches or routers (not shown) that host board 103 or the otherboard serializes together into higher data rate electrical signals.Pluggable optical module 105 via photonics 127 converts the electricalsignals to an optical signal for further transmission into network 190via optical fiber 104 when the optical transceiver 101 is in a normaloperating mode (not a testing mode).

Pluggable interface 125 includes connection points 121A-121N(collectively referred to as “connection points 121”) and pluggableinterface 115 includes connection points 111A-111N (collectivelyreferred to as “connection points 111”). When pluggable optical module105 couples to host board 103, connection points 121 can mate withcorresponding connection points 111 to provide a continuous electricalpath for data transmission and reception between pluggable opticalmodule 105 and host board 103.

For example, as illustrated in more detail with respect to FIG. 2,photonics 127 of pluggable optical module 105 can include lasers andphase- and amplitude-modulating optical hardware to mix pairs of datasignals received from host board 103 to produce a single set of opticalsignals for transmission. Photonics 127 can also include the opticalhybrid mixers to convert the received optical signal into the pairs(e.g., in-phase and quadrature) of data signals, referred to as I and Qdata signals, for transmission to host board 103.

In some examples, in addition, the modulated light wave in onepolarization may be multiplexed with another modulated light wave inanother polarization, which may be orthogonal to the previous one, toproduce a polarization-multiplexed (PM) signal, such as PM-M-QAM, anexample of which is PM-QPSK where M=4. The polarizations of the lightwave signals may be chosen to be orthogonal to allow for a simplepolarization beam splitter and digital signal processing to be used forpolarization demultiplexing when photonics 127 receives data fromnetwork 190. For example, PM-QPSK modulation uses two input electricaldata signals per polarization to impart the complex information on theoptical carrier. The electrical signal for each polarization (e.g.,polarization X and polarization Y) contains a pair of in-phase (I) andquadrature (Q) data signals that represent the complex data waveform. Inthis context, polarization X and polarization Y refer to any appropriatetype of orthogonal polarizations such as for example vertical andhorizontal polarization, or clockwise circular polarization andcounterclockwise circular polarization.

For example, when the optical transceiver 101 is in a normal operatingmode (not a testing mode), photonics 127 of pluggable optical module 105receives a downstream optical signal from network 190 via optical fiber104. In this example, the downstream optical signal is modulated inaccordance with the PM-QAM (e.g., PM-QPSK) modulation scheme. Photonics127 converts the downstream optical signal into two pairs of I and Qoptical data signals, and converts the two pairs of I and Q optical datasignals to two pairs of I and Q electrical data signals (referred to aspairs of I/Q electrical data signals for ease of reference). In thisexample, the pairs of I/Q electrical data signals together representmagnitude and phase information for the received signal. Photonics 127transmits the pairs of I/Q electrical data signals to host board 103 viathe electrical path provided by the mating of connection points 121 toconnection points 111.

In long-haul optical communication systems, optical amplifiers can beused to compensate the attenuation introduced by an optical fiber(s).While the optical signal(s) is boosted, noise is also added which causesthe decrease of optical signal noise ratio (OSNR) and increase of biterror ratio (BER). High BER can prevent or limit forward errorcorrection (FEC) from correcting errors, which reduces or limits thetransmission distance. Therefore, BER vs OSNR is monitored to measurethe performance of an optical transceiver.

FIG. 2 is a block diagram illustrating examples of photonics within apluggable optical module of an optical transceiver, according to anembodiment. The optical transceiver 201 can be structurally andfunctionally similar to the optical transceiver 101 described withregards to FIG. 1. The optical transceiver 201 can include a pluggableoptical module 205 and a host board 203 operatively coupled to thepluggable optical module 205. The components of the pluggable opticalmodule 205 are illustrated for PM-QAM modulation. Pluggable opticalmodule 205 can include additional, fewer, or different components thanthose illustrated here without limiting the applicability of thisdisclosure. In alternate examples, pluggable optical module 205 caninclude different configurations and/or components to achieve PM-QAMmodulation.

The transmit photonics of pluggable optical module 205 can include laser234, controller 235, beam splitter (BS) 236, drive amplifiers 237A-237D,optical modulators 238A and 238B, and polarization beam combiner (PBC)240. PBC 240 is operatively coupled to the network 290 via an opticallink 230 (e.g., an optical fiber). In some situations, the pluggableoptical module 205 receives electrical data signals from the host board203, converts the electrical data signals into modulated optical signals(e.g., a PM-QAM modulated optical signal). The PBC 240 then outputs suchmodulated optical signals to the network 290 via the optical link 230when the optical transceiver 201 is in a normal operating mode (not atesting mode).

Drive amplifiers 237A-237D can amplify the voltage level of theelectrical signals of the pair of in-phase (I) and quadrature (Q) datasignals for each polarization, i.e., XI′, XQ′, YI′, and YQ′ datasignals, output by host board 203. Laser 234 can be any type of laserthat is usable for high bit rate optical signal transmission, typicallya narrow linewidth laser in the 1550 nm wavelength range (so-calledC-Band), but can be tuned to any wavelength. Optical amplifiers(included in network 290 and not shown) operating in the same wavelengthrange can allow pluggable optical module 205 to transmit the modulatedoptical signal a relatively far distance appropriate for long-haulcommunication.

Beam splitter (BS) 236 receives the light from laser 234 and splits thelight into (at least) two paths. Each one of optical modulators 238A and238B receives light from one of the paths. Optical modulators 238A and238B modulate the light on the respective paths with respective I/Qelectrical data signal pairs. Optical modulators 238A and 238B may bereferred to as IQ modulators or Cartesian modulators. In the example ofFIG. 2, optical modulator 238A receives the XI′ and XQ′ electrical datasignals from DAs 237A and 237B and modulates the light received fromlaser 234 via BS 236 to form a complex modulated optical signal,modulated in both magnitude and phase, forming a first QAM signal.Optical modulator 238B receives YI′ and YQ′ electrical data signals fromDAs 237C and 237D and modulates the light received from laser 234 via BS236 to form a complex modulated optical signal, modulated in bothmagnitude and phase, forming a second QAM signal.

A polarization rotator (not shown in FIG. 2.) can rotate thepolarization of either X arm or Y arm by 90 degrees so that thepolarization states from the X arm and the Y arm are orthogonal.Polarization beam combiner (PBC) 240 receives the polarized andmodulated optical signals output from optical modulators 238A and 238B(each at a different polarization) and combines the polarized andmodulated optical signals into a single optical signal. For instance,the optical QAM signals from optical modulators 238A or 238B are thenmultiplexed in (nominally orthogonal) polarizations using PBC 240. Forexample, PBC 240 combines the received QAM optical signals intonominally orthogonal polarizations, i.e., into a singlepolarization-multiplexed (PM) optical signal with one component having Xpolarization and another component having Y polarization, and transmitsthe PM-QAM optical signal to network 290 via optical link 230.

In other words, optical modulator 238A modulates the light wavegenerated by laser 234 based on the XI′ and XQ′ electrical signals.Optical modulator 238B modulates the light wave generated by laser 234based on the YI′ and YQ′ electrical signals. PBC 240 combines the firstand second optical signals to form a polarization-multiplexed opticalsignal.

The controller 235 can be or can include a general purpose processor, afield-programmable gate array (FPGA), an application specific integratedcircuit (ASIC), a combination thereof, or other equivalent integrated ordiscrete logic circuitry. The controller 235 can include a processor(not shown in figure) and a memory (not shown in figure). The memory canbe, for example, a random-access memory (RAM) (e.g., a dynamic RAM, astatic RAM), a flash memory, and/or so forth. In some implementations,the memory can include or store, for example, a process, application,and/or some other software modules (stored and/or executing in hardware)or hardware modules.

The processor (not shown in figure) can be or can include a generalpurpose processor, a field-programmable gate array (FPGA), anapplication specific integrated circuit (ASIC), a combination thereof,or other equivalent integrated or discrete logic circuitry. Theprocessor (not shown in figure) can be configured to, for example, writedata into and read data from the memory, and execute the instructionsstored within the memory.

When the optical transceiver 201 is in a normal operating mode (not atesting mode), the receiver photonics of pluggable optical module 205can receive a polarization-multiplexed optical signal from the network290. For instance, the receiver photonics of pluggable optical module205 can include a variable optical attenuator (VOA) 244, a polarizationbeam splitter (PBS) 246, a photo diode 280, optical hybrid mixers 248Aand 248B, and balanced photo-detectors (PDs) 250A-250D. In someimplementations, there can be eight photo-detectors forming four pairsof balanced photo-diodes. Thus, each pair is illustrated as onephoto-detector in FIG. 2.

The variable optical attenuator (VOA) 244 can include, for example, aMach-Zehnder Modulator (MZM), a micro-electromechanical system (MEMS),or other technology that varies an optical signal amplitude as afunction of an applied electrical signal (not shown). The photo diode280 can be operatively coupled to VOA 244 and can measure the receiveroptical power (ROP) of the optical signals from the VOA 244. Themeasured ROP value can be calibrated by the controller 265 to accountfor the insertion loss of the optical hybrid mixers 248A-248B and theVOA 244. The ROP value, after being calibrated in some instances, can besent to the host board 203 for predication of the BER v OSNRperformance. In some situations, the ROP can also be measured by one ofthe photo-detectors (PDs) 250A-250D. The PBS 246 can receive an opticalsignal from the VOA 244 and split the received optical signal into first(XI and XQ) and second optical signals (YI and YQ) with nominallyorthogonal polarization.

Each of the optical hybrid mixers 248A and 48B can mix the respectiveoptical signals from PBS 246 with a local oscillator signal and outputoptical data signals representing respective in-phase (I) andquadrature-phase (Q) components of the PM-QAM modulated signal to thePDs 250A-250D. For example, optical hybrid mixer 248A outputs XI and XQoptical data signals. Optical hybrid mixer 248B outputs YI and YQoptical data signals. In some examples, optical hybrid mixers 248A and248B may be 90 degree optical hybrid mixers.

PDs 250A-250D can receive respective optical signals of the XI, XQ, YI,and YQ optical data signals and convert these optical signals intoelectrical signals (e.g., the XI, XQ, YI, and YQ data signals that thehost board 203 receives). As noted above, photo-detectors 250A-250D maybe composed of a pair of nominally balanced photo-diodes. Atransimpedence amplifier (TIA) element (not shown) for eachphoto-detector may be used to convert the photo current from thephoto-diode(s) to a voltage representation.

In this manner, when the optical transceiver 201 is in a normaloperating mode (not a testing mode), the receive photonics of theoptical transceiver 201 can convert the PM-QAM modulated optical signalinto electrical I and Q data signal pairs (e.g., the XI, XQ, YI, and YQdata signals) for further processing by host board 203. For example,host board 203 receives the XI, XQ, YI, and YQ electrical data signalpairs from photo detectors 250A-250D through the mating betweenconnection points of the pluggable optical module 205 (such as 121A-121Nin FIG. 1) and connection points of the host board 203 (such as111A-111N in FIG. 1).

When the optical transceiver 201 is in a testing mode, such as during amanufacturing operation, a calibration operation, a trouble-shootingoperation or an upgrading operation, the transmitter of the opticaltransceiver 201 can be configured to be connected to the receiver of theoptical transceiver 201 through one of an optical switch 239 integratedwithin the optical transceiver, or an external loop-back connection 270.Such optical switch 239 and the loop-back connection 270 can facilitatethe measurements of the receiver optical power (ROP) value and the biterror rate (BER) value. Details of the methods of the measurements aredescribed below.

FIG. 3 shows a graph indicating examples of an original BER value and afitted BER value as a function of an original OSNR value at differentROP values, according to an embodiment. In some situations, BER vs OSNRresults can be dependent on receiver optical power (ROP). As describedin FIG. 2, the ROP value can be measured by the photo diode 280 or oneof the photo-detectors (PDs) 250A-250D. As shown in FIG. 3, the x-axis301 is the original OSNR value, and the y-axis 305 is the original BERvalue and the fitted BER value. The original BER values are representedby dots, and the fitted BER value are represented by solid lines in thefigure. The curves 321-324 show the fitted BER values as a function ofthe OSNR value at different ROP values. The curve 321 shows the fittedBER values as a function of the OSNR value when the ROP is equal to −22dBm. The curve 322 shows the fitted BER values as a function of the OSNRvalue when the ROP is equal to −18 dBm. The curve 323 shows the fittedBER values as a function of the OSNR value when the ROP is equal to −10dBm. The curve 324 shows the fitted BER values as a function of the OSNRvalue when the ROP is equal to 0 dBm. The power of the electricalsignals is proportional to ROP value of the power of the opticalsignals. Thus, at lower ROP (e.g., when ROP is −22 at 321), theelectrical signal power is lower which can lead to lower signal noiseratio (SNR) and higher BER. At higher ROP (e.g., when ROP is −10 at323), the electrical signal power is higher which can lead to higher SNRand lower BER. Therefore, to characterize the BER vs OSNR performance,multiple measurements over different ROP can be performed.

An analytical model (1) was used to predict (or estimate) theperformance of BER vs OSNR.

$\begin{matrix}{{OSNR}_{calib} = \frac{10^{\hat{}}( {{OSNR}^{dB}/10} )*{resBW}}{2*{baudRate}}} & (1) \\{\frac{1}{S\; N\; R_{total}} = {\frac{1}{\kappa} + \frac{1}{O\; S\; N\; R_{calib}}}} & \; \\{{BER}_{fit} = {{erfc}( \sqrt{\eta\; S\; N\; R_{total}} )}} & \; \\{{{Minimize}\mspace{11mu}\theta} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}( \frac{{BER}_{fit} - {BER}_{measure}}{{BER}_{fit}} )^{2}}}} & \; \\{{to}\mspace{14mu}{solve}\mspace{14mu}\eta\mspace{14mu}{and}\mspace{14mu}\kappa} & \; \\{{BER}_{floor} = {{erfc}( \sqrt{\eta*\kappa} )}} & \;\end{matrix}$

Here, the “baudRate” is the baud rate of electrical signal including aFEC overhead, “resBW” is 12.5 GHz (0.1 nm) against which the OSNR ismeasured, and “η” and “κ” are two fitting parameters. The analyticalmodel solves two fitting parameters “η” and “κ” by minimizing the errorbetween the measurement result and the curve-fitting result. “κ” isrelated to the noise floor and “η” is related to filter mismatching orbandwidth. The curve-fitting results of the model are shown as solidlines in FIG. 3. As shown in the figure, the fitted curves (dashedcurves in the figure) substantially fit with the measurement results(solid curves in the figure). For each ROP value, however, a differentset of “η” and “κ” values is needed. Thus, it is desirable to extend theexisting model with consideration of ROP so that a single set ofparameters can be extracted. Those parameters can be stored inElectrically Erasable Programmable Read-Only Memory (EEPROM) of theoptical transceiver, or a database of the optical transceiver, or otherstorage medium of the optical transceiver. In one example, the networkmanagement compute device and/or multi-layer optimization tool canimprove the design and manufacture of an optical transceiver bypredicting (or estimating) and monitoring the BER vs OSNR performance.

FIG. 4 is a graph representing examples of the correlation between theBER vs OSNR curve and the BER vs ROP curve, according to an embodiment.The BER vs OSNR curve and BER vs ROP curve can be intrinsicallycorrelated. In BER vs OSNR measurement, the noise increases while thepower of the optical signal remains constant when OSNR decreases. In BERvs ROP measurement, the power of the optical signal decreases while thenoise remains constant when ROP decreases. Both scenarios can lead tothe decrease of SNR and the increase of BER. This correlation can beseen in FIG. 4. Here the symbols 405 represents the BER vs OSNR curve at0 dBm ROP value, with X axis 401 being the OSNR value (in dB). The curve406 represents the BER vs ROP curve, with X axis 401 being the ROP value(in dB) plus a constant shift (e.g., 45 dB). Those two curvessubstantially overlap, indicating strong correlation between BER vs OSNRcurve and BER vs ROP curve. The constant shift in ROP value can be theOSNR value from the coherent transmitter at 0 dBm output power. Thisconstant shift is around the same for the CFP2-ACO module from the samevendor, but it could be different for the module from a differentvendor.

Given this correlation between BER vs OSNR curve and BER vs ROP curve,the analytical model of BER vs OSNR curve can be modified to be appliedto BER vs ROP curve. The modified model (2) can introduce a newparameter, p, for the consideration of ˜45 dB OSNR at 0 dBm ROP value.In some instances, p can be equal to 31622 (45 dB),

$\begin{matrix}{{ROP}_{calib} = \frac{{10\hat{}( {{ROP}^{dB}/10} )}*{resBW}}{2*{baudRate}}} & (2) \\{{\frac{1}{\kappa} + \frac{1}{\rho^{lin}{ROP}_{calib}}} = \frac{1}{{SNR}_{total}}} & \; \\{{BER}_{fit} = {{erfc}( \sqrt{\eta\;{SNR}_{total}} )}} & \; \\{\theta = {\frac{1}{n}{\sum\limits_{i = 1}^{n}( \frac{{BER}_{fit} - {BER}_{measure}}{{BER}_{fit}} )^{2}}}} & \; \\{{BER}_{floor} = {{erfc}( \sqrt{\eta*\kappa} )}} & \;\end{matrix}$

The modified model (2) can extract the fitting parameters of “η” and “κ”from the BER vs ROP measurement, then use the fitting parameters topredict (or estimate) BER value at certain OSNR value. An expanded model(3) expands the analytical model of the BER vs OSNR performance with theconsideration of the influence by the ROP.

$\begin{matrix}{{\frac{1}{\kappa} + \frac{1}{\rho^{lin}{ROP}_{calib}} + \frac{1}{{OSNR}_{calib}}} = \frac{1}{{SNR}_{total}}} & (3) \\{{BER} = {{erfc}( \sqrt{\eta\;{SNR}_{total}} )}} & \;\end{matrix}$

Similarly, the fitting parameters of “η” and “κ” can be extracted fromthe BER vs ROP measurements. These fitting parameters can be furtherused to predict (or estimate) the BER value at certain OSNR values. Insome instances, fitting parameter η can be between 0.82 and 0.9, andfitting parameter κ can be between 14.6 and 20.7. In some instances,fitting parameter η is equal to 0.85 and fitting parameter κ is equal to17.6.

FIG. 5 shows examples of comparison between the measurement result ofthe BER vs OSNR curve and the prediction (or estimation) result of theBER vs OSNR curve, according to an embodiment. The x-axis 501 representsthe original OSNR. The y-axis 502 represents the measured BER and thepredicted BER. The solid curves (512, 505, 506, 517) represent themeasurement result of the BER vs OSNR curve, and the dashed curves (514,515, 507, 504) represent prediction (or estimation) result of the BER vsOSNR curve. Curves 512 and 514 respectively represent the measurementresult and the prediction result of the BER vs OSNR performance when ROPis equal to −22 dBm, at 513. Curves 505 and 515 respectively representthe measurement result and the prediction result of the BER vs OSNRperformance when ROP is equal to −18 dBm, at 511. Curves 506 and 507respectively represent the measurement result and the prediction resultof the BER vs OSNR performance when ROP is equal to −10 dBm, at 510.Curves 517 and 504 respectively represent the measurement result and theprediction result of the BER vs OSNR performance when ROP is equal to 0dBm, at 508.

As shown in FIG. 5, the measurement results substantially overlap withthe prediction (or estimation) result when ROP is from 0 dBm (508) to−22 dBm (513), 507-511. When ROP is below −22 dBm, some deviation can beobserved at high OSNR region (not shown in the figure). At low OSNRregions where most long-haul optical communication systems operate at,the two curves are substantially aligned.

The modified model allows abstraction of performance of a coherentoptical transceiver through a set of fitting parameters η, κ, and ρ. Thefitting parameters η and κ can be determined by BER vs ROP measurementduring a testing mode of the optical transceiver such as a manufacturingoperation, a calibration operation, a trouble-shooting operation or anupgrading operation (discussed further below). Fitting parameter ρ canbe determined during a design verification testing process. In oneimplementation, these fitting parameters can be stored in EEPROM of theline-card. The network management compute device or the multi-layeroptimization tool can extract those parameters, and perform networkmanagement and optimization. For example, in the initial planning andoptimization of a network, the network management compute device canobtain the fitting parameters from EEPROM of the optical transceiver. Amulti-layer optimization tool (e.g., at the processor 117 of the hostboard 103 in FIG. 1) can be used to predict the BER vs OSNR performance,and determine where to place the particular line-card based on thetransmission distance. When the network is in a normal operating mode, adynamic re-routing of a certain signal may be necessary due to eventssuch as, but not limited to, fiber cut and equipment failure. After thesignal is re-routed, the OSNR of the signal may change due to adifferent transmission distance. In this scenario, the networkmanagement compute device or the multi-layer optimization tool canpredict the BER vs OSNR performance based on those fitting parameters,and determine whether this new route is feasible for a particularoptical transceiver.

The optical transceiver can modulate optical signals in one of a set ofmodulation formats including polarization-division-multiplexedquadrature-phase-shift-keying (PDM-QPSK), andpolarization-division-multiplexed quadrature-amplitude-modulation(PDM-QAM). The optical transceiver can modulate optical signals in8-QAM, 16-QAM, 32-QAM, or 64-QAM.

The modified model can provide simple measurements of the BER vs OSNRperformance of the optical transceiver via an external optical loop-backconnection (such as the loop-back connection 270 in FIG. 2) or theinternal optical switch (such as switch 239 in FIG. 2) to connect thetransmitter of the optical transceiver to the receiver of the opticaltransceiver. In this embodiment, the external optical loop-backconnection or the internal optical switch allow the measurements of theBER vs ROP curve during a testing mode of the optical transceiver suchas a manufacturing operation, a calibration operation, atrouble-shooting operation or an upgrading operation. A photo diode (ora power meter, e.g., PDs 250A-250D) can measure the ROP values and sendthe ROP values to the processor (such as the processor 117 in FIG. 1).The processor can be configured to measure a set of BER values of adigital modulated signal (such as the XI′ XQ′ YI′ and YQ′) at an inputport of the optical transceiver. In some embodiments, the processor canmeasure a set of BER values in response to a varying ROP parameterthrough an external variable optical attenuator (VOA, e.g., 244 in FIG.2) operatively coupled to the optical transceiver or an internal VOAintegrated in the optical transceiver. The processor can then constructor generate a BER vs ROP curve based on the measured ROP values and theBER values. In other embodiments, the processor can measure a set of BERvalues in response to a varying control parameter other than the ROP.Therefore, the processor can construct or generate a BER vs the varyingcontrol parameter curve.

In most situations, the ROP values and BER values can be measured inreal-time when the optical transceiver is operating. The fittingparameters η and κ can be previously determined in the initialmanufacturing phase and stored in the EEPROM of the optical transceiver.The fitting parameters η and κ can be periodically re-calibrated whenthe optical transceiver is in a testing mode.

Fitting parameters η and κ can be determined from such BER vs ROPmeasurements. Fitting parameter ρ can be determined during designverification testing process. Using the modified model with the fittingparameters η, κ, and ρ, the BER vs OSNR can be predicted (or estimated).In another embodiment, such measurements and prediction can facilitate anetwork operator to determine performance of the optical transceiver.For example, during a maintenance window, once the BER vs ROP ismeasured, a current set of fitting parameters η and κ is determined, andthe network operator can compare the current set of fitting parameterswith the original values of the fitting parameters stored in EEPROM.Based on the comparison results, the network operator can determinewhether any performance degradation of the optical transceiver ispresented and can raise alarm (or request maintenance or replacement)accordingly.

It is advantageous to use such embodiments to predict BER vs OSNRperformance of an optical transceiver because no hardware is required inaddition to an optical switch integrated within the optical transceiver,or an external loop-back connection. This method is also independent ofmodulation format. Most vendors can adopt this approach into theirexisting products without alterations, which facilitate theinter-operation and openness of optical line systems. It is alsoadvantageous to predict BER vs OSNR based on the embodiments for routeplanning and network management purpose.

FIG. 6 shows graphs of examples of BER vs OSNR curves at different ROPvalues representing accuracy of the BER vs OSNR prediction, according toan embodiment. The x-axis 601 represents the original OSNR. The y-axis602 represents the measured BER and the predicted BER. The solid curves(612, 605, 606, 617) represent the measurement result of the BER vs OSNRcurve and the dashed curves (614, 615, 607, 604) represent prediction(or estimation) result of the BER vs OSNR curve. Curves 612 and 614respectively represent the measurement result and the prediction resultof the BER vs OSNR performance when ROP is equal to −22 dBm, at 613.Curves 605 and 615 respectively represent the measurement result and theprediction result of the BER vs OSNR performance when ROP is equal to−18 dBm, at 611. Curves 606 and 607 respectively represent themeasurement result and the prediction result of the BER vs OSNRperformance when ROP is equal to −10 dBm, at 610. Curves 617 and 604respectively represent the measurement result and the prediction resultof the BER vs OSNR performance when ROP is equal to 0 dBm, at 608.

In most long-haul optical communication systems, the BER level beforeforward error correction (i.e., pre-FEC BER) is around 1e-3. The FEClayer of the optical transceiver can correct certain amount of BER andreport the pre-FEC BER. In one embodiment, the pre-FEC BER can be usedto reversely predict the OSNR. As shown in FIG. 6, the OSNR monitoringaccuracy is <0.2 dB over 0 dBm ROP (608) to −22 dBm ROP (613).

FIG. 7 shows graphs of examples of Q² factor (in dB value) (or Q2factor) vs chromatic dispersion (CD) values of an optical transceiver,according to an embodiment. The x-axis 701 represents the CD value,while the y-axis on the left 702 represents the Q² factor and the y-axison the right 703 represents the OSNR value. The Q² factor can be afunction of the BER value: Q² (dB)=10*lg₁₀(2*erfc⁻¹(BER){circumflex over( )}2) where erfc⁻¹ is the inverse error function, and lg₁₀ is logarithmfunction with base 10. In some implementations, during opticaltransmission over a relatively long distance, additional opticalimpairments, for example, the chromatic dispersion (CD), polarizationmode dispersion (PMD) with first order PMD being differential groupdelay (DGD), and carrier frequency offset (CFO) can be generated. Acoherent optical transceiver (such as the optical transceiver 101 inFIG. 1) together with a digital signal processing (DSP) chip can recoverthe signals even with a large amount of CD and PMD present. In somesituations, the DSP chip can be included in the processor (such asprocessor 117 in FIG. 1) of the host board (such as host board 103 inFIG. 1). The coherent optical transceiver can report the estimated CDvalues. The DGD value is linearly proportional to the PMD value. In somesituations, additional BER degradation is present due to these opticalimpairments. As shown in the figure, the Q²-factor 702 is approximatelylinearly proportional with CD value 701, which can be represented by acoefficient α.

FIG. 8 shows graphs of examples of the Q² factor (in dB value) vs DGDvalue of an optical transceiver at different ROP values, according to anembodiment. The x-axis 801 represents the DGD value, while the y-axis onthe left 802 represents the Q² factor and the y-axis on the right 803represents the OSNR value. As shown, the Q² factor function 802 isapproximately linearly proportional with DGD value function 801, whichcan be represented by a coefficient β. A model (4) to predict OSNR frompre-FEC BER, ROP, CD, DGD is developed. During design verificationtesting, these coefficients, α and β as in model (4) can be determinedand used to further improve the OSNR monitoring accuracy.

$\begin{matrix}{{{ROP}_{calib} = \frac{{10\hat{}( {{ROP}^{dB}/10} )}*{resBW}}{2*{baudRate}}},{{SNR}_{total} = \frac{{{erfc}^{- 1}( {BER}^{{pre} - {FEC}} )}^{2}}{\eta}}} & (4) \\{\frac{1}{O\; S\; N\; R_{calib}} = {\frac{1}{{SNR}_{total}} - \frac{1}{\kappa} - \frac{1}{\rho^{lin}{ROP}_{calib}}}} & \; \\{{OSNR}_{est}^{dB} = {{10*{\log_{10}( \frac{2*{baudRate}*{OSNR}_{calib}}{resBW} )}} + {\alpha*{CD}} + {\beta*{DGD}}}} & \;\end{matrix}$

FIG. 9 is a flow chart illustrating a method of an improved OSNRperformance measurement of an optical transceiver, according to anembodiment. This method can be implemented at a processor of an opticaltransceiver (e.g., processor 117 in FIG. 1) or at a processor of a layerhigher than the layer of the host board (such as host board 103 in FIG.1). In some instances, the processor of a layer higher than the layer ofthe host board can be a processor of a management and control layer ofthe wavelength division multiplexing system. The method includes, whenthe optical transceiver is in a testing mode, measuring, via a photodiode, a receiver optical power (ROP) value and sending the ROP value tothe processor at 902. The testing mode of the optical transceiver caninclude a manufacturing operation, a calibration operation, atrouble-shooting operation or an upgrading operation. During thetesting, a transmitter of the optical transceiver is connected to areceiver of the optical transceiver through an optical switch integratedwithin the optical transceiver, or through an external loop-backconnection. The optical transceiver can modulate optical signals in oneof a set of modulation formats includingpolarization-division-multiplexed quadrature-phase-shift-keying(PDM-QPSK), and polarization-division-multiplexedquadrature-amplitude-modulation (PDM-QAM). The optical transceiver canmodulate optical signals, for example, in 8-QAM, 16-QAM, 32-QAM, or64-QAM.

At 908, the processor measures a bit error rate (BER) value of a digitalmodulated signal at an input port of the optical transceiver. Theprocessor can measure a set of BER values at different ROP values andderive fitting parameters η, κ, and ρ based on the BER vs ROP curve.Fitting parameter η represents a bandwidth parameter. Fitting parameterκ represents a noise-floor parameter. In some instances, fitterparameter “ρ” can be equal to 31622 (45 dB), for the consideration of˜45 dB OSNR at 0 dBm ROP value. Such fitting parameters are stored inthe optical transceiver, such as stored in EEPROM, database or otherstorage medium of the optical transceiver.

At 910, the processor determines an estimated optical signal noise ratio(OSNR) value at the input port of the optical transceiver based on theROP value and the BER value. Based on the measured BER values and thefitting parameters η, κ, and ρ, the OSNR values can be estimated. Thus,the BER vs OSNR curve can be determined and used to evaluate theperformance of the optical transceiver. In some implementations, theprocessor receives a signal indicating a chromatic dispersion (CD)value, a differential group delay (DGD) value, and a carrier frequencyoffset (CFO) value in a normal operation. The processor uses the CDvalue, the DGD value, or the CFO value to improve accuracy of theestimated OSNR.

At 912, the processor sends a signal indicating the estimated OSNR valuesuch that a planned route is selected for sending data signals throughwithin the optical transceiver based on the estimated OSNR value. Ifthere is a decreased performance based on the estimated BER vs OSNRperformance of the optical transceiver, the processor can sends a signalto alert the performance degradation for route planning and networkmanagement. In another embodiment, such measurements and prediction canbe used to assist or facilitate a network operator to determineperformance of the optical transceiver. For example, during amaintenance window, once the BER vs ROP is measured, a current set offitting parameters η and κ is determined, and the network operator cancompare the current set of fitting parameters with the original valuesof the fitting parameters stored in EEPROM. Based on the comparisonresults, the network operator can determine whether any performancedegradation of the optical transceiver is presented and can raise alarm(or request maintenance or replacement) accordingly.

Some embodiments described herein relate to a computer storage productwith a non-transitory computer-readable medium (also can be referred toas a non-transitory processor-readable medium) having instructions orcomputer code thereon for performing various computer-implementedoperations. The computer-readable medium (or processor-readable medium)is non-transitory in the sense that it does not include transitorypropagating signals per se (e.g., a propagating electromagnetic wavecarrying information on a transmission medium such as space or a cable).The media and computer code (also can be referred to as code) may bethose designed and constructed for the specific purpose or purposes.Examples of non-transitory computer-readable media include, but are notlimited to: magnetic storage media such as hard disks, floppy disks, andmagnetic tape; optical storage media such as Compact Disc/Digital VideoDiscs (CD/DVDs), Compact Disc-Read Only Memories (CD-ROMs), andholographic devices; magneto-optical storage media such as opticaldisks; carrier wave signal processing modules; and hardware devices thatare specially configured to store and execute program code, such asApplication-Specific Integrated Circuits (ASICs), Programmable LogicDevices (PLDs), Read-Only Memory (ROM) and Random-Access Memory (RAM)devices. Other embodiments described herein relate to a computer programproduct, which can include, for example, the instructions and/orcomputer code discussed herein.

Examples of computer code include, but are not limited to, micro-code ormicroinstructions, machine instructions, such as produced by a compiler,code used to produce a web service, and files containing higher-levelinstructions that are executed by a computer using an interpreter. Forexample, embodiments may be implemented using imperative programminglanguages (e.g., C, Fortran, etc.), functional programming languages(Haskell, Erlang, etc.), logical programming languages (e.g., Prolog),object-oriented programming languages (e.g., Java, C++, etc.) or othersuitable programming languages and/or development tools. Additionalexamples of computer code include, but are not limited to, controlsignals, encrypted code, and compressed code.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Where methods described above indicate certain eventsoccurring in certain order, the ordering of certain events may bemodified. Additionally, certain of the events may be performedconcurrently in a parallel process when possible, as well as performedsequentially as described above.

What is claimed is:
 1. An apparatus, comprising, an optical transceiverconfigured to be operatively coupled to a network, the opticaltransceiver including a variable optical attenuator (VOA) and aprocessor operatively coupled to the VOA, the VOA configured to vary areceiver optical power (ROP) parameter to obtain a plurality of ROPvalues, the processor configured to measure, in response to the VOAvarying the ROP parameter, a plurality of bit error rate (BER) valuesassociated with the plurality of ROP values, the processor configured toestimate a plurality of optical signal noise ratio (OSNR) values basedon the plurality of BER values and the plurality of ROP values, theprocessor configured to send a signal indicating the plurality of BERvalues as a function of the plurality of OSNR values during at least oneof a manufacturing operation, a calibration operation, atrouble-shooting operation or an upgrading operation.
 2. The apparatusof claim 1, wherein: the optical transceiver is configured to modulateoptical signals in one modulation format from a plurality of modulationformats including polarization-division-multiplexedquadrature-phase-shift-keying (PDM-QPSK), andpolarization-division-multiplexed quadrature-amplitude-modulation(PDM-QAM).
 3. The apparatus of claim 1, wherein: the first opticaltransceiver is configured to modulate optical signals in one modulationformat from a plurality of modulation formats including 8-QAM, 16-QAM,32-QAM, and 64-QAM.
 4. The apparatus of claim 1, wherein: a transmitterof the optical transceiver is configured to be connected to a receiverof the optical transceiver through at least one of an optical switchintegrated within the optical transceiver, or an external loop-backconnection.
 5. The apparatus of claim 1, wherein: the processor isconfigured to receive a signal indicating a chromatic dispersion (CD)value, a differential group delay (DGD) value, and a carrier frequencyoffset (CFO) value, the processor is configured to improve accuracy ofthe plurality of OSNR values based on at least one of the CD value, theDGD value, or the CFO value.
 6. The apparatus of claim 1, wherein: theVOA includes one of an external VOA operatively coupled to the opticaltransceiver or an internal VOA integrated in the optical transceiver. 7.The apparatus of claim 1, wherein: the processor configured to send asignal indicating the plurality of BER values as a function of theplurality of OSNR values for route planning and network management.
 8. Amethod, comprising, varying, at a variable optical attenuator (VOA) ofan optical transceiver, a receiver optical power (ROP) parameter toobtain a plurality of ROP values; measuring, at a processor of theoptical transceiver and in response to varying the ROP parameter, aplurality of bit error rate (BER) values associated with the pluralityof ROP values; estimating, at the processor, a plurality of opticalsignal noise ratio (OSNR) values based on the plurality of BER valuesand the plurality of ROP values; and sending, from the processor, asignal indicating the plurality of BER values as a function of theplurality of OSNR values during at least one of a manufacturingoperation, a calibration operation, a trouble-shooting operation or anupgrading operation.
 9. The method of claim 8, further comprising:modulating, at the optical transceiver, optical signals in onemodulation format from a plurality of modulation formats includingpolarization-division-multiplexed quadrature-phase-shift-keying(PDM-QPSK), and polarization-division-multiplexedquadrature-amplitude-modulation (PDM-QAM).
 10. The method of claim 8,further comprising: modulating, at the optical transceiver, opticalsignals in one modulation format from a plurality of modulation formatsincluding 8-QAM, 16-QAM, 32-QAM, and 64-QAM.
 11. The method of claim 8,further comprising: connecting a transmitter of the optical transceiverto a receiver of the optical transceiver through at least one of anoptical switch integrated within the optical transceiver, or an externalloop-back connection.
 12. The method of claim 8, further comprising:receiving, at the processor, a signal indicating a chromatic dispersion(CD) value, a differential group delay (DGD) value, and a carrierfrequency offset (CFO) value; and improving, at the processor, accuracyof the plurality of OSNR values based on at least one of the CD value,the DGD value, or the CFO value.
 13. The method of claim 8, wherein: theVOA includes one of an external VOA operatively coupled to the opticaltransceiver or an internal VOA integrated in the optical transceiver.14. The method of claim 8, further comprising: sending, from theprocessor, a signal indicating the plurality of BER values as a functionof the plurality of OSNR values for route planning and networkmanagement.
 15. A non-transitory processor-readable medium storing coderepresenting instructions to cause a processor of an optical transceiverhaving a variable optical attenuator (VOA) to: vary a receiver opticalpower (ROP) control parameter to obtain a plurality of ROP values;measure, in response to varying the ROP parameter, a plurality of biterror rate (BER) values associated with the plurality of receiveroptical power (ROP) values; estimate a plurality of optical signal noiseratio (OSNR) values based on the plurality of BER values and theplurality of ROP values; and send a signal indicating the plurality ofBER values as a function of the plurality of OSNR values.
 16. Thenon-transitory processor-readable medium of claim 15, furthercomprising: receive, at the processor, a signal indicating a chromaticdispersion (CD) value, a differential group delay (DGD) value, and acarrier frequency offset (CFO) value; and improve, at the processor,accuracy of the plurality of OSNR values based on at least one of the CDvalue, the DGD value, or the CFO value.
 17. The non-transitoryprocessor-readable medium of claim 15, further comprising: send, fromthe processor, a signal indicating the plurality of BER values as afunction of the plurality of OSNR values for route planning and networkmanagement.
 18. The non-transitory processor-readable medium of claim15, wherein code to send the signal includes code to send the signalduring at least one of a manufacturing operation, a calibrationoperation, a trouble-shooting operation or an upgrading operation.